Fails bot checks on "XCord" bungee servers (bug where bot falls after receiving abilities flying flag?)

[ItzDerock]

• The FAQ doesn't contain a resolution to my issue

Versions

• mineflayer: 4.17.0
• server: XCord (Bungeecord fork) (unknown version)
• node: 21.4.0

Detailed description of a problem

XCord advertises an Advanced Fake Lobby Check which prevents mineflayer clients from joining the server:

Tests whether the client responds to collision, protocol checks, gravity, etc, in a virtual lobby before allowing them into the lobby server. Only takes around ~3 seconds to complete!

Pakkit capture

Left = mineflayer (1.20.1)
Right = Vanilla MC 1.20.1

The first difference is that Vanilla always sends settings before custom_payload, while mineflayer does the opposite. The contents of these packets are the same. I don't think the server cares about this though.

Next is a simple gravity check, looks like they both pass this test (it looks like the server sets Y and expects client to reach a target Y)

The next test is where it starts to differ:

After receiving an abilities packet with flags 6 (0x11, which are the Flying and Allow Flying bits enabled), the mineflayer player starts dropping while the Vanilla client floats in the air.
The server repeats this test a few times before deciding to kick the mineflayer bot.

[ItzDerock]

if you want me to upload the pakkit export, I can, just lmk

[rom1504]

Thanks for the detailed report.

Do you want to fix it ?

I would recommend these steps

1. Make sure we handle the abilities packet in entities.js and store the properties it provides
2. Update physics.js to fly instead of fall if the fly ability is turned on
3. Manually test it works
4. Ideally set up an automated test for this anticheat to make sure it keeps working in the future

[u9g]

The falling half of this is that nothing in mineflayer reads the abilities packet at all, so the flying bit in your capture never reaches physics. #4083 tracks the packet (bot.abilities, bot.entity.flying) and PrismarineJS/prismarine-physics#142 makes simulatePlayer hover instead of applying gravity when it is set, matching Player.travel (vertical velocity damped 0.6, getFlyingSpeed in place of the 0.02 of a falling player).

The settings-before-custom_payload ordering you noted at the top is a separate thing and is not addressed there.